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The ancient parasite, described today (June 27) in the journal
Current Biology, had a mouth and body smaller than older fleas,
but larger than modern-day pests. The new species,
Saurophthyrus exquisitus, may be a transitional species
that could shed light on why modern-day bloodsucking parasites
evolved to become smaller and take dainty, unobtrusive bites.

Ancient pests

Last year, Chungkun Shih, a visting professor at Capital Normal
University in Beijing, and his colleagues discovered the
oldest known flea. The ancient parasites, known as
Pseudopulicidae, were unearthed in 165-million-year-old
sediments in northeastern China.

Pseudopulicidae had huge, 0.8 inch-long (2 centimeters)
bodies with long tubes for sucking blood and sharp, sawlike
teeth. The males also had completely external genitalia. Those
ancient pests likely fed on the blood of thick-skinned, feathered
dinosaurs that lived during the Jurassic
Period. [ Dinosaur
Fleas! Photos of Paleo Pests ]

"The flea needed to cut through the thick skin to get to the
blood, and they could do that harm without the host knowing it,"
Shih told LiveScience.

By contrast, modern-day fleas have bodies that are five to 10
times smaller, and have much smaller mouths, completely hidden
genitals and longer legs for jumping.

Shih and his colleagues were excavating in the same region when
they uncovered three specimens of the new species,
Sauropthyrus exquisitus. The ancient pest had a body
size in between the oldest and modern-day fleas, growing up to
0.4 inches (1 cm) long. It also sported partially hidden
genitalia, and a thin, relatively small sucking tube for drawing
blood; it also lacked the fierce teeth of its older relative.

In addition, the new species had longer legs and short, stiff
bristles on its body.

The researchers hypothesize that fleas originally evolved to
feast on thick-skinned dinosaurs, so piercing the skin was the
main challenge.

But as dinosaurs evolved, so did their parasites. The
pterosaurs, or flying reptiles, that lived in the same region
during that Cretaceous
Era had much thinner skins. As a result, Sauropthyrus
exquisitus adapted to deliver less painful bites, "so they
would be harder to detect by the host," Shih said.

The bristles on its body may have helped the paleo pest cling to
hairs on an animal's body to hide. And the longer legs and
partially internal genitalia may also have enabled greater
jumping ability, Shih said.

"You can imagine if you have something sticking out, it's hard to
move around," Shih said, referring to the genitalia.

The new discovery suggests the parasites co-evolved with their
hosts in a way to balance their bloodsucking and hiding
abilities. Mammals have even thinner, more sensitive skin than
pterosaurs, which makes the ability of fleas to take dainty,
unobtrusive sips and jump away from a deadly swat especially
important, Shih said.